A Ga-rich InGaN quantum well layer comprising 10% or less of InN is mainly used to form a UV light source using nitride semiconductors. It is known that as the light emission wavelength is reduced, the light emission efficiency is lowered.
Generally, in case of a green or blue light source in the visible light range using nitride semiconductors, it is possible to obtain a high light emission efficiency in spite of a high defect density in a thin layer due to the absence of a proper substrate. This is because of the formation of a local carrier energy level caused by phase separation and composition nonuniformity of InN in the InGaN quantum well layer. It is known that this effect can be increased as the compositional rate of InN is increased.
However, in case of a UV light source, the InN composition in the InGaN quantum well layer is smaller than that in the visible light source, the local carrier energy level is rarely formed and thereby, the light emission efficiency is lowered. Also, as compared to the green or blue light source, the difference of energy level between an InGaN quantum well layer and a capping layer (or barrier layer) is small and thereby, the carrier confinement effect is reduced, causing a decrease in the light emission efficiency.
For these reasons, it is impossible to have a high light emission efficiency in the conventional UV light source using a Ga-rich InGaN quantum well layer with an InN composition of 10% or less.